1. Field of the Invention
The present invention relates to a mixer circuit in a CATV UP/DOWN converter, and particularly to a mixer circuit in a CATV UP/DOWN converter comprising a double-balanced mixer including a diode bridge.
2. Related Art
The input frequency used in a conventional CATV UP/DOWN converter was in a range of from 57 MHz to 570 MHz in America by way of example. This input frequency has, however, a tendency that the reception band is widened to a range of from 57 MHz to 860 MHz.
However, a mixer circuit in a first stage used at present is constituted by a double-balanced mixer including a diode bridge as shown in FIG. 3, and a radio-frequency input signal is sent to the secondary side of a transformer 72 through transformers T1 and T2. A local oscillation frequency signal is supplied to an intermediate terminal of the secondary side of the transformer T2 through a capacitor Cp1. The radio frequency is converted into an intermediate frequency in this portion. Then, the two terminals on the output side of the transformer T2 are connected to connection terminals Pa and Pb respectively. The anode of a diode D1 and the cathode of a diode D2, which are connected reversely, are connected to the connection terminal Pa, and the cathode of a diode D3 and the anode of a diode D4, which was connected reversely, are connected to the connection terminal Pb.
Between the cathode of the diode D1 and the anode of the diode D3, a capacitor C1 is connected for making the respective electric potentials of the cathode of the diode D1 and the anode of the diode D3 equal to each other in terms of radio frequency (shortcircuiting the diodes D1 and D3 in terms of radio frequency). Also between the anode of the diode D2 and the cathode of the diode D4, a capacitor C3 is connected for making the respective electric potentials of the anode of the diode D2 and the cathode of the diode D4 equal to each other in terms of radio frequency.
In addition, the cathode of the diode D1 is connected to one terminal of the capacitor C2 through a point M, and further connected to one input terminal Ta of a transformer T3. On the other hand, the anode of the diode D3 is connected to a coil L3 through a point N. The other terminal of the coil L1 is connected to the other terminal of the capacitor C2, and further grounded through a resistor R1. Further, the output terminal of the transformer T3 on the side connected to the one input terminal Ta is connected to one terminal of a parallel circuit of a capacitor C5 and a coil L5 through a coil L3.
In addition, the cathode of the diode D4 is connected to one terminal of the capacitor C3 through a point S, and further connected to the other input terminal Tb of the transformer T3. In addition, the anode of the diode D2 is connected to a coil L2 through a point P. The other terminal of the coil L2 is connected to one terminal of a capacitor C4. In this case, the capacitor C4 is connected in parallel with the above-mentioned resistor R1.
On the other hand, the output terminal of the transformer T3 on the side connected to the other input terminal Tb is grounded. In such a configuration, the frequency characteristic of the mixer circuit does not extend, and the intermediate-frequency characteristic also deteriorates. Particularly, in a band of from 57 to 860 MHz, the frequency characteristic as a double-balanced mixer also depends on the capacitor C1 for making the respective electric potentials of the cathode and the anode of the diodes D1 and D3 equal to each other in terms of radio frequency, and the capacitor C3 for making the respective electric potentials of the anode and the cathode of the diodes D2 and D4 equal to each other in terms of radio frequency. Generally, the capacitance of this capacitor C3 is about 1,000 PF. With such a capacitance, however, there was a problem that the frequency characteristic on the high frequency side, including the frequency characteristic of a balun, the characteristic of circuit configuration (portions A and B) to the input terminals Ta and Tb of the transformer T3 in the post-stage of the diode bride, and the characteristic of the diodes, does not extend by additional inductance and capacitance caused by the parts such as the diode bridge, the transformers, and so on, and the pattern on a board on which the parts are connected, if the input frequency becomes higher.